1. Field of the Invention
The present invention relates to centrifugal pumps and more particularly relates to centrifugal pumps having housings formed by nonmetallic, corrosion resistant materials.
2. Prior Art
Pumps constructed from nonmetallic, corrosion resistant materials, notably plastics or fiber reinforced plastic (FRP) materials, are generally known in the prior art and have been constructed principally to reduce the cost of pumps used in applications where highly corrosive fluent materials are pumped. Pumps constructed from plastic materials have been used as alternatives to pumps constructed from substantially more expensive materials, such as stainless steel, to handle fluent materials which are chemically active. Examples of these substances are acidic or alkaline liquids used in various industrial processes such as metal treating, washing, etc.
Typical plastic pump constructions have been highly resistant to corrosion and deterioration as a result of exposure to chemically active pumped liquids but in some environments the pumped liquids have contained abrasive particulate matter which, when entrained in the pumped fluid, has tended to quickly erode the plastic material at locations in the pump where flow rates and velocities are high. Centrifugal pumps constructed from plastic materials have been particularly susceptible to erosion at the pump cutwater and along radial surfaces extending from the impeller eyes to volute walls extending about the impellers. In this type of environment pumps constructed from stainless steel have continued to be employed since stainless steel exhibits good corrosion and wear resistance. Stainless steel pumps are quite expensive and for that reason have been undesirable, notwithstanding their advantageous physical properties.
While plastic pump construction materials are fairly rugged and durable, they have been subject to relatively significant flexure when subjected to substantial pressure differentials. In particular, pump housings formed by assemblies of molded plastic parts have been subject to flexural stresses which distort the parts of the assemblies which has resulted in adjacent housing parts tending to separate. The plastic pump housing parts have also generally exhibited relatively low hoop strengths which has resulted in the peripheral regions of housing members tending to expand when subjected to differential pressure forces. Flexure of the plastic housing material has resulted in a tendency for these pump housings to leak during operation of the pumps. When corrosive or toxic fluids were being handled any such leakage was quite undesirable.
In order to minimize the effects of bending and hoop stresses on the plastic pump parts a number of measures have been proposed for strengthening or reinforcing the plastic pumps. In some instances the plastic parts have been reinforced by metal backing plates or specialized multipart clamping assemblies which were secured to the pump housings, or by the use of relatively large numbers of individual fasteners, or combinations of these expedients. Other approaches have suggested employing relatively heavy and/or specially-shaped housing wall sections (including molded in webs, ribs, bosses, etc). In general these approaches tended to complicate the pump constructions, thus increasing the complexity of manufacture and their cost as well as complicating maintenance and servicing of the pumps in the field.
Where plastic walls were specially formed to provide reinforcing structures such as heavy wall sections, webs, bosses, ribs, etc., and/or specially formed joints between assembly parts, the molded plastic parts were costly to fabricate. The molding apparatus for producing such parts was complex and expensive. For example, a molding technique used in constructing many prior art plastic pumps employed heated, matched metal molds in which the pump housing sections were formed.
Manufacturing some pumps required that a number of plastic housing parts be placed in jigs or fixtures in appropriate relative orientations and then secured together by fasteners, multipart clamping assemblies and the like. This was sometimes a tedious and time-consuming operation, particularly where the proper relative orientation of the pump parts was difficult to perceive. Furthermore, when a pump in the field required servicing, to the extent the pump could be disassembled for replacement or repair of internal parts, reassembly of the pump was frequently a difficult task.
Plastic or FRP pump housing members were sometimes provided with openings having molded or tapped threads for receiving threaded fasteners. The molded or tapped threads tended to strip when the housing members distorted under pressure. Stripped housing threads frequently lead to separation of pump housing parts during use of the pump and consequent leakage. To avoid the problem, tapped metal sleeves have been molded into the walls of the plastic pump housing members but the sleeves further complicated the pump fabrication processes and/or assembly procedures.